Steam-stripping of sulfuric acid containing coke-forming impurities



E. J. HENLEY STEAM-STRIPPING 0F' SULFURIC ACID CONTAINING July 11, 1961COKE-FORMING IMPURITIES Filed Sept. 22, 1958 mrOO .vom @I IOoZ ERNEST J,HENLEY ATTORNEY United States larent 2,992,070 STEAM-STRIPPING FSULFURIC ACID CON- TAINING COKE-FORMING IMPURITIES Ernest J. Henley, NewYork, N.Y., assignor to American Cyanamid Company, New Y'ork, N.Y., acorporation of Maine Filed Sept. 22, 1958, Ser. No. 7 62,524 3 Claims.(Cl. 23-173) This invention relates to an improvement in processeswherein sulfuric acid containing coke-forming organic impurities isstripped by the injection of superheated steam and more particularly toa method of controlling the particle size and agglomeration of the cokeformed during such stripping processes.

Sulfuric acid having a substantial content of polymerizable hydrocarbonsand other tarry impurities must be dealt with as a Waste material in anumber of industries.

Representative by-products of this character are spent alkylation acids,such as those resulting from the production of high octane gasoline andyfrom the fo-rmation of diarylethanes by the condensation of acetylenewith aromatic hydrocarbons. yFor example a typical spent alkylation acidproduced in the condensation of butylene with paraffins such aslpropanes and butanes may contain about 83-90% of sulfuric acid, 4-8% ofunsaturated hydrocarbons and 5-8% of carbon, the remainder being freeoil. A spent sulfuric acid from the production of diarylethanes asexemplified by the process of U.S. Patent No. 2,734,928 may containabout 6575% of sulfuric acid, about l0-25% of arylsulfonic acid, about36% of tar together with water and metallic mercury liberated from thecatalyst. These and other similar spent sulfuric acids are frequentlytreated by stripping with superheated steam, which decomposes andvolatilizes their more volatile impurities -and converts the heavierresidues into coke. Thus, the stripping of spent sulfuric acid from theproduction of diarylet'hanes by means of superheated steam is de scribedin U.S. Patent No. 2,832,810.

When such steam-stripping processes are operated on a commercial scalethe formation of large masses of coke in the stripping kettleconstitutes a serious problem. Such coke masses, which may reach thesize of l -foot or more in diameter, can cause the stalling or breakingof agitators and other damage to the equipment and in smaller sizes maycause the plugging of valves and pipe lines. It is a principal object ofthe present invention to provide a method of controlling the particlesize of the coke formed in such steam-stripping processes and to preventits agglomeration into large masses.

I have found that this object can be accomplished by mixing finelydivided non-reactive solids with the spent sulfuric acid before thesuperheated steam is injected therein. The presence of such finelydivided solids prevents agglomeration of the coke particles into massesand ensures the formation of a granular coke that will not damage orclog the equipment and that can be readily separated from the clarifiedacid by filtration when the steam-stripping process is completed.

Although agglomeration of the coke into large masses can be prevented bythe presence of finely divided solids of almost any character, providedthey are not reactive with the sulfuric acid, I have found that the bestresults are obtained with solids having a porous and absorbentcharacter. Solids of this nature, such as finely divided diatomaceousearth of the ykind used as a filter precoat or filter aid, appear toabsorb and adhere to the coke particles as they are formed and thusprevent their agglomeration into larger masses. Coke produced by theprocess is also absorbent and may be used. The qu-antity of diatomaceousearth or other finely divided solid to be used may vary through a widerange, depending on the charfr ICC acter of the carbonaceous impuritiespresent in the spent sulfuric acid, but will ordinarily be from about1/2 to 4 times the weight of the tars or other coke-forming impuritiesin the spent sulfuric acid.

The operating conditions to be maintained during steamstripping of spentsulfuric acid to which diatomaceous earth or other finely divided solidshave been added are ordinarily about the same as those employed in theabsence of added solids. The spent acid-added solids mixture is heatedto stripping temperatures, which are ordinarily within the range ofabout 140-190 C. depending largely on the strength of the sulfuric acidbeing stripped. Superheated steam having a temperature higher than thestripping temperature and usually within the range of l300 C. isinjected into the acid preferably with mechanical agitation. Theinjection of steam is continued until the desired degree ofdecomposition and coking of the impurities has been attained; this mayrequire from about 1 hour to 6 hours or longer and is also dependent toa considerable extent on the strength and temperature of the sulfuricacid. The volatiles removed from the acid may be recovered by condensingthe steam if desired.

After completion of the stripping the sulfuric acid containing thesuspended solids and coke -is preferably cooled and filtered. It is animportant advantage of the invention that this filtration is materiallyaided by t-he relatively uniform particle size and granular condition ofthe coke produced in the presence of added solids such as diatomaceousearth. Any suitable type of filter may be used such as a plate and framefilter press or a rotary filter since ordinarily the filter kcake isopen and easily drained. A clarified sulfuric acid suitable forfortification and reuse is ordinarily obtained, while the coke may beburned for its fuel value or for recovery of the added finely dividedsolids.

The invention will be further described with reference to theaccompanying drawing, the single figure of which is a flow sheetillustrating its application in the recovery of a spent sulfuric acidobtained in the alkylation of toluene with acetylene. On this drawing analkylator indicated generally by reference numeral 1 is charged withtoluene, sulfuric acid and mercuric sul-fate catalyst and with acetylenegas -at the rates indicated in the following specific example and ismaintained at 0-10 C. by a coolant such as liquid ammonia introducedthrough line 2. The resulting mixture of ditolylethane, excess toluene,mercury, spent sulfuric acid and toluene sulfonic acid together with thetars formed during the reaction is discharged through line 3 into asettling tank or decanter 4, where it separates by gravity into `anupper alkylate layer and a lower layer of spent sulfuric acid. The spentacid, which contains the toluene sulfonic acid and much olf the mercuryand tarry impurities, is discharged through line 5 into an acid stripper6 Where it is mixed with diatomaceous earth and stripped by theinjection of superheated steam. During the stripping much of the toluenesulfonic acid is decomposed into toluene and oxides of sulfur which arevolatilized along with the mercury and volatile components of the tar.The steam and vapors pass through line 7 into a scrubber 8 where theyare Washed with a recirculating stream of cold aqueous sodium hydroxidesolution. The uncondensed gases escape through a vent 9, the mercury isrecovered from the base of the scrubber in a mercury pot 10, and thetoluene is separated in a separator 11 and returned to the toluenesupply tank. The oxides of sulfur react with the aqueous causticsolution and are recovered as sodium sulfate.

When stripping of the spent acid is complete the contents of the acidstripper are cooled, preferably by vacuum cooling, and are dischargedthrough line 12 to a filter 13. Because of the granular condition of thecolze a continuous rotary lter of the type illustrated diagrammaticallyon the drawing may be used. The claried sulfuric acid obtained asfiltrate from this lter is collected in a storage tank 14 while the cokeis discharged for combustion or other disposal.

The quantities of materials and operating conditions in a preferredembodiment of this process are given in the following specific example.It should be understood, however, that this example is given primarilyfor the purpose of illustrating the invention, and should not beconstrued as a limitation thereof.

Example The alkylator 1 was operated continuously for a time such that1500 lbs. of acetylene were reacted with a mixture of 37,400 lbs. oftoluene, 4766 lbs. of sulfuric acid, 250 lbs. of water and 13 lbs. ofmercurio sulfate while maintaining a reaction temperature of 5 C. Theresulting mixture was discharged -into the decanter 4 where it separatedinto an upper layer containing the ditolylethane product and excesstoluene and a lower acid layer. This layer weighed about 6500 lbs. andcontained about 65% of H2504, about 22% of toluene sulfonic acid, 7% ofwater, 6% of tar land about 5 lbs. of mercury.

The lower layer was drawn off and charged into the acid stripper 6 andagitated as 780 lbs. of diatomaceous earth of a grade used as a lter aidwere added and mixed. Superheated steam having a temperature of about200 C. was then introduced, While continuing the agitation, until atemperature of 171-173 C. was reached, after which the batch wasstripped by continuing the steam addition for 2 hours while maintainingthis temperature. A total of 1440 pounds of steam was used. The toluene,oxides of sulfur, mercury and steam were passed through line 7 into thescrubber 8 where they were recovered by contact with a recirculatingstream of aqueous 10% sodium hydroxide solution. The mercury settled bygravity into the mercury pot 10 while the oxides 4l of sulfur reactedwith the alkali. About 1000 lbs. of toluene were recovered as an upperlayer in the separator 11 and returned to the toluene storage tank.

During the stripping process there was no agglomeration of the coke intolarge masses. A granular mixture of coke and diatomaceous earthparticles having a small and uniform particle size was suspended in thestripped acid. This was readily separated from the acid by liltration onthe rotary filter 13.

What I claim is:

l. A method of decomposing and removing tarry impurities in a spental'kylation sulfuric acid while preventing the formation of agglomeratedcoke masses therein which comprises mixing finely divided non-reactivesolids with said acid and then injecting Superheated steam there- 2. Amethod of decomposing and removing tarry impurities in a spentalkylation sulfuric acid while preventing the formation of agglomeratedcoke masses therein which comprises mixing diatomaceous earth with saidacid and then injecting Superheated steam therein.

3. A method according to claim 2 in which the spent alkylation acid isobtained by reacting an Ialkylbenzene with acetylene in the presence ofa dispersion of a mercury salt in strong sulfuric acid, settling theresulting alkylate mixture into layers, and drawing off the bottom layercontaining the spent acid.

References Cited in the file of this patent UNITED STATES PATENTS2,624,697 Clouse et -al Jan. 6, 1953 2,709,676 Krebs May 3l, 19552,832,810 Sm-olin Apr. 29, 1958 2,863,823 Moser Dec. 9, 1958 2,886,514Gn'fn et al May 12, 1959 2,895,904 Jones et al July 2l, 1959

1. A METHOD OF DECOMPOSING AND REMOVING TARRY IMPURITIES IN A SPENTALKYLATION SULFURIC ACID WHILE PREVENTING THE FORMATION OF AGGLOMERATEDCOKE MASSES THEREIN WHICH COMPRISES MIXING FINELY DIVIDED NON-REACTIVESOLIDS WITH SAID ACID AND THEN INJECTING SUPERHEATED STEAM THEREIN.